Asset folder colors were first brought up in 2021 and you are the second person to mention them, so I don’t think this is exactly high demand, but if you’d like to take a peck at this yourself,
Colors are loaded here:
https://github.com/YellowAfterlife/GMEdit/blob/8719161f306b84201e1e2e9b8232b80b6fc31dc2/src/yy/YyLoader.hx#L106-L122
Colors are applied here:
https://github.com/YellowAfterlife/GMEdit/blob/8719161f306b84201e1e2e9b8232b80b6fc31dc2/src/yy/YyLoader.hx#L46-L57
Treeview context menus are populated here:
https://github.com/YellowAfterlife/GMEdit/blob/8719161f306b84201e1e2e9b8232b80b6fc31dc2/src/ui/treeview/TreeViewMenus.hx#L317
So you would add a new sub-menu with color selection that adds/updates/removes an item in the project’s asset_browser.json and calls applyAssetColour() for the item.